The present invention relates to power switching circuits and, in particular, to driver circuits for driving power switching circuits. In particular, the present invention relates to a bootstrap capacitor power supply refresh circuit utilized in a driver circuit for driving a power switching circuit, for example, a half bridge power switching circuit driving a load.
Bootstrap capacitors are often employed in driver circuits to provide an additional fkiatubg voltage from an existing power supply or from an existing voltage source or from a pulsed signal source. In particular, as shown in FIG. 1, which shows a driver IC for driving a half bridge switching circuit comprising two power MOSFETs M1 and M2 driving a load, as shown there, the half bridge transistors M1 and M2 are arranged between a voltage source VBAT and ground. The voltage supply is also provided to the driver IC (IC) at a voltage input VCC to provide power to the driver IC. VBAT may be a different, higher voltage than VCC, or it may be the same voltage. Typically, VCC is different than VBAT because of high voltage spikes on VBAT. VCC may be derived from VBAT by an internal or external regulator circuit. The driver IC includes a driver HO and a driver LO which drive the respective high side and low side transistors M1 and M2. In addition, an external bootstrap capacitor CBOOT is provided between a terminal VB and the terminal VS which is coupled to the load and which is the common connection between the transistors M1 and M2. A supply voltage VBOOT is thus provided across the bootstrap capacitor. A diode D is disposed either internally to the IC or external to it to allow the bootstrap capacitor CBOOT to charge up from VCC when transistor M2 is on. The bootstrap supply is used to supply a votlage to the high side driver.
A problem arises when there is a permanent conduction of transistor M1. By “permanent conduction” is meant a relatively lengthy on time for transistor M1, for example, more than 200 usecs. In such a circumstance, transistor M2 is off and accordingly, the bootstrap capacitor cannot charge through transistor M2 during such operation. Accordingly, the circuit of FIG. 1 cannot be used if transistor M1 has relatively lengthy on times, for example conduction times greater than 200 usec. Transistor M2 will simply not be on long enough to allow the bootstrap capacitor to charge up and therefore provide a power source to circuits either internal to the IC or external to it that are powered by VBOOT.
It is therefore necessary to provide a solution to this problem, and particularly one that is useful in automotive applications.